Demodulator for frequency and amplitude modulation



Aug. 14, 1945.

' E. H. LANGE I DEMODULATOR FOR FREQUENC Y AND AMPLITUDE MODULATION Filed Feb. 23, 1943 INVENTOR.

Patented Aug. 14, 1945 UNITED STAT ES ,IOFFICE' DEMODULATOR FOR FREQUENCY- AND AMPLITUDE MODULATION Edward H.1iange, Balsam, Ma; 7 Application February 23,19 43, Serial No. 476,854 j I 16 Claims.

This invention pertains to thermionic'demodulator devicesfor converting the pattern of frequency-modulation'of a high-frequency carriervoltage into corresponding output voltages, and more particularly to apparatus for compensating undesirable modifications introduced or superposed upon these output voltages by extraneous amplitude-modulation of the carrier voltage, either at random, or by desired amplitude-modulated signal intelligence; and for utior tubes 'ahead'oif the frequency demodulator for amplitude-limitation; and providing" selective amplification of either the frequency-demodulation for .the amplitude-demodulation by the thermionic push-pull amplifier employed as part of the balanced niodulatorgalso to providesim'uljtaneous demodulation of both frequency-modulated signals and amplitude-modulated signals at lizing the identical component parts andoperating structureof the demodulator device, foralso demodulating amplitude-modulation.

The principal object of this invention is to provide such'a demodulator, having both'high sensitivity of conversion, and economy of, component parts, and to provide economy of space to accommodate these parts,

Another object of thisinvention is to provide such a demodulator capable of converting frei quency deviations above and below a centre frequency into respectivedeviations of positive and negative voltages, directly proportional to the magnitude of thefrequenoy deviations; without the necessity of regulating the'impressed voltage upon the demodulator to a minimum determined by theweakest signal, to eliminate amplitudemodulation; also to provide operation of ,the frequency-demodulator from a variable norm of impressed voltage upon the demodulator, and to provide a selection of operating magnitudes of impressed voltage. l

Another object of this invention is ,to provide in a frequency-demodulator, a useful unithe separate respective output terminals for these demodulations. Y fThese objects, and others, are hereafter set forthlin further detail in. the following specifi cation and appended claims, and will bebetter understood by reference to the drawing.

In the drawing,

Fig.v 1 illustrates a demodulator in accordance with the, principles ,of this invention, having a thermionic compensator tube, double-diode, resonant: circuitrzand :choke-coil connected to the double-diode, a thermionic push-pull amplifier, connections to the output circuit. of the compensator tube from the push-pull. amplifier, and

selector means for selecting; either ,-frequencydemodulation' or. amplitude-demodulation for amplification by th push-pull amplifler; the resonant circuit andconnected choke-coil being capacitively :coupled with the impressed? modulated directional control-voltage, which variesin direct proportion to the amplitudes of the alternating currents supplied to, the input-side of the demodulator, and which is independent uof the frequency deviations from icentrerfrequency, of

these alternating currents, overithe entire useful range of such frequency deviation. I

A further object .of this invention is to utilize this unidirectional control-voltage with suitable thermionic compensating meansv to provide a conductance relationship with negative bias voltcompensating voltage for a balanced ,modulator connected to the output-side of the frequencydemodulator, and of proper magnitude and phase relationship to compensate for the undesirable effect upon the output of the frequency-demodulator caused by variations in magnitude of the alternating currents supplied to the frequencydemodulator. I

Still another object of this invention is to provide a demodulator, requiring no thermionic tube age upon the .cont'rol-"grid of the compensator tube, for providing "the'outputvoltage of the 'compensatortub'e utilized 'in the input circuit of "the push-pull ampliflen'to' yield the necessary compensation for the undesirable effect of amplitude-variations'upon the output'voltageot the frequency-demodulator.

Fig.' 4 illustrates in vector-diagram form certain unique relationships found' to exist with reference to voltage and current proportions, in the j structures oifFlg; 1 and "Fig. 2, and' employed to yield the important unidirectional 'control voltagejof this invention; independent of frequency grid 81, and suppressor-grid 84 connected to the cathode 88 by conductor 89. Connected to the cathode'88 is the, negative voltagebias-resistance 82', shunted by condenser 8|. Terminals 90 and 9|, connected respectively to control-grid 81 and bias-resistance 82", are input terminals for sup plying modulated alternating voltages to the le modulators. voltage, connected to ground l from the mega;

to the negative terminal 85 and ground l5 of source 12, through the condenser 15. The junction 59 between condenser 51 and coil 64, is connected to the terminal 10 of choke coil 1 I through condenser 65. The junction 69 between coils 63 and 64, is connected by conductor 66 to the anode l3 of double-diode l2, and the junction 68 be tween condenser 51 and coil 58 is connected by conductor 61; to' anode of double-diode l2. Connected to the junction b between'resistances 2] and 28, by conductor d, is a terminal of the "resistance 31; the other terminal of resistance At 12 is a source of unidirectional.

tive terminal of the source; positive voltage from v the source is supplied to thescreen-grid 86.by' the variable conductor 92, and the anode 83 is connected to the positive terminal of the source 72, Fig i through the choke-0.011;, and in Fig.-.2,- through inductance coilfH-r Connections to;.the-tube 82 are illustrated only for the pure pose of continuity and for; illustrating the utilization ofqa-common-source 1,2, of this invention, therebeingmany well. understood types of linkhe, principal-:terminals-.,'liI-85, 1,, .Fig.2,., flthis inventibil, and, capab e of sunplyine, modulated alternatingflvoltages to these terminals.

At, [.2 is shown a double-diode',- having I cathl odes i4 a d fz nd anodes .13 and 25. sh nt d across'theloathodes' +26, are the equal resistances ZLLZ B'Lin seriGS the oommonjunction b between these resistances being, connected to ground 15 by gconductor (Z. Condenser 29 is shunted; across resistance; 21 and condenser; 30 i s n edi 1 55 esistan e 2 21M. l are 2 are equal resistances connected in; series, and having rent of both diodes of the double-diode thefco nmon connection ,or junctiona which is connected its grounclfli'through the secondary boil 5 'ofjthetransformer"39%5. At 3'2' is'a conidenser'co necting the c'athode l4 with the terminal of resistance; I opposite the junction e, and

'likewiseat aflcondenser connecting cathode "26 the'terminal" of resistance 2 opposite the Junction [At Fiitihdli'rjare terminals connected respectivelyto the above-said terminals of resistancez and resistance I; these terminals bein g" output volta'ge terminals -=for" the alternat- :injgj voltages developed by frequency modulati'on iimpres'sed'iu'pon the 'demodulators At'Ti's-atherinionic compensator tube, having thecathode' 96; anode- 9, suppressor-grid In connected' td 'cathodetfi by conductor Hla, oontrol- V grid H and" screen-grid s connected to a positive voltage upon the source 12 by variable conductor :94, the control-grid 1 H" being "connected by the variable contact'or -ali'ito-iresistancei 3:1. The anode 9 of tube 1, is connectedto the source 12 through the primary'winding 39 of transformer 59 -5, by the variable conductor 93: connected toa positive voltage upon '12; The cathode 9S connectedcby the variablecontactor ,14 to the bias resistancez'm, which is shunted across a part of the "source 12; at the negative terminal ofthe '31- being-connected to the terminal 62 of choke- C0i1' 56',1 Wh'lCh terminal is capacitively coupled with groundl5 through condenser 15. The resistance ,31' carries the total unidirectional curl2. Shuntedacross resistance 31 is the condenser 38; also shunted across resistance 31 is the condenser 49 in serieswith,resistance .6, one terminalof re stance 6 being-connected ,to ground I5. and QQnduct r d, the other -te rminal being connected to; terminal ,Ai. f The. terminals A1 and ground l5'fserye' as output terminals for. the alternating voltages developed, by amplitude-modulation im- Rre 'sed upon.- the f demodulator.

, 1 nt 4| and 42 are thermionic tubes of apush nullfamplifier. ,,Tub'e 4| 'has' anode 44, sup

' i erid 4, ,ic'ont 91-srid 4 and s een rid l kewise tube 42 has anode 43, suppressor,- eir 2; "con rol-g id. 5L d e re Cathodes' 4fi and' 4'l of tubes, 4l and 42 respectively, are each c't' nnected' to. the common junction, 511 as area l soj'the suppressor-grids 4'5 and 5 25,; the junction being connected by v conductor 55c and v'a conductor 95' to a positive voltage'uponthesource 12 Screen grids 49 and 53 are connected bythe common conductor 94d and the variable onductor-same, positive voltage upon the sourc -12.? Shunted across the anodes 44 and 43 'areYtheresistances 22 and 24 in series, the junctionR between these resistances being connected by conductor 93%; and variable conductar 'g3 'to 'posit'ive voltage upon the source 12. The resistances-[land I'8 are connected in series,

the common junction h betweenthese resistances I j,be connected; with A and blade,55 with; A1,

source, and: which has one terminal. connected transformer 3-9,-;- 5,=is; the variable resistance 35,

variable-bycontaot r 35a: 1'5' Referring-toriig il; at 59:40, is ahresonant circuit-having the coils :64, 6-3, and 58 and var- The choke-coil 56 has one i'able "condenser-.51.,

terminal connected to ihe j nction 60 between coils 63 and 58, and the other terminal connected it being. understood that the blades 54 and 55 are locked togetherin. ajmanner well understood to move jointly The resistances 22 and 24 are fequal 'likewise thf resistances I1 and [8 are equal,

and thetubes v:4! and 42.are understood to have substantially identical characteristics, I

.rfIfhejcoilsj3y5fll'and 64, of the resonant circuit ffifi fifljhareproportionedso that at exact resonanceiof this circuit, to the centre frequency of the frequency-modulatedflalternating currents, the voltages -.,between terminals 69 and 60 and between, terminals 8 and 6.0., are equal. It will be understood that the resonant circuit 5,960, can

condenser 51, 01'. a single coil with condenser 51 c nnected toits terminals, the terminals as, so,

and thereon being so placed as to provide the .above stated equality'of voltages. In Fig. 2, the resonant circuit 63a-58a5la, has the terminal 89a connected by conductor 86 to anode l3 of double-diode l2, and the terminal 68a connected by conductor 61 to anode 25 of double-diode l2. The terminal of resistance 31 opposite the ground-connected terminal of this resistance is connected to a terminal of the chokecoil 550, the other terminal of choke-coil 560. being connected to the half-inductance point 60a upon the single inductance coil made up of the coils tau and 58a in series. The variable condenser 51a is connected across the terminals 59a and 68a of the series-connected coils 63a and 58a. The variable condenser 16 is shunted across the inductance coil 11; the inductance coil 11 has mutual inductance with coils 63a and 58a, and the junction 60a upon these coils is capacitively coupled with the terminal a, by condenser 13. The terminals F2 and F1, and terminal A1, are for use with the switching means 54-55 of Fig. 1, and with the push-pull amplifier connected to said switching means. 7

Havin illustrated certain circuit connections embodying this invention, the principlesof this invention are hereafter discussed to point out the important features, and with reference tothe above-disclosed circuit connections. l

In devices hitherto employed for demodulating frequency-modulated alternating currents, it has been necessary to eliminate any variations in amplitude of the impressed alternating currents,

'since such devices are also responsive to amplitude variations; without such elimination the resultant response to frequency-modulation would be distorted by extraneous voltages unrelated to frequency-modulation. Devices hitherto employed, and requiring one or moretubes to accomplish satisfactory elimination of amplitude variations, have employed a saturation principle, wherein alternating currents impressed inexcess of a certain minimum value are unable'to produce at the output side alternating currents larger than the minimum value. Such devices result in the employment of signals determined by the weakest signal-strength, and but little gain is contributed by the tube or tubes used 'toso limitthe signal amplitudes. It is therefore necessary tov provide, by means of the amplifying apparatus preceding such a'limiting device,an excess of amplification such that the weakest signal will upon "amplification, equal the constant norm fixed by the characteristics of the limiting device, and no benefitis thus derived from thesignal-strengths which exceedsuch a fixed minimum. In the devices of this invention, operation of the demodulators takes place upon a selectable norm of input-signal amplitude, modifications of the amplitude of input signals above and below thenorm being compensated in their effect upon the output voltages of the frequency-demodulator, by combining .with these output voltages a compensating 'voltage. This combination is accomplished through the utilization of a balanced modulator, in which the compensating voltage is introduced as the common voltage upon both-control-grids, and in which the compensating voltage is appropriately related to the changes in the ratio of the norm of alternating input currents to actual input alternatingcurrents, and employed to crossmodulate the output voltages of the frequency demodulator in such a manner asto cancel the undesired modifications of these output voltages previously introduced by the modifications of amplitude of the input alternating currents.

In order to provide operation of the irequencyternating currents impressed upon the demodulator, and which is at the same time, independent of the frequency deviations from centre-frequency, for,' the entire range of such deviations employed upon the demodulator. The discovery of means for utilizing such a control-voltage in a frequency-demodulator, to compensate for variations of alternating current amplitudes, byproducing therefrom a cross-modulation factor inversely related to said amplitudes, and employing said factorin a balanced amplifier connected to said demodulator, is an important feature of this invention. r

r The principle of operation of the unidirectional control-voltage will be first pointed out with reference tof Fig. 1, wherein a resonant circuit 59-60 is in series with. a, substantially pure inductive reactance ,of'cholre-coil 56; The stopping condensers 65 and 15 have negligible impedance .at the operating .carrier-frequencies, in relation degrees from the alternating current through'the choke-coil 56. The resonant circuit 59,60 ,is

sharply resonant to the centre-frequency, ,tlie coils 63, 64, and 58 havinga very high reactanceresistance ratio. Thus, at exact resonance for the centre-frequency, the alternating Voltage across resonant circuit 59 60 is in phase with the alternating current passing through chokecoil 56, and is therefore degrees displaced in phase from voltage 56c. At A is shownthe vectorvoltage across the coil 6 3 of resonant circuit 59 -60, and at B is shownthe vector voltage across the coil 58 of the resonant circuit, at exact resonance to centre-frequency. When the voltage across coil 63 is directed toward terminal 60, the voltage across coil 58 is directed away .from terminal 60. The voltage impressed across the anode-cathode l3ll4 is determined by the resultant, voltage across terminals 69 and likewise the voltage impressed across anode-cathode 25-2li is determined by the resultant voltage-across terminals 6862. At exact resonance, these resultant'voltages are equal, and 'areshown by .vectors E1 and E2. The corresponding unidirectional currents through the diodes, are pro .portional to the magnitudes ofthese" resultant voltages, and equal unidirectional currentsflow from the. cathodes through resistancesil and 28 to the junction b, there being no resultant'unidirectional voltage across resistances 2l -2B in series, that, is. between the cathodes I4 and.

Condensers 29 and 30 havea negligible impedance inrelation to, resistances 2 7 or ,28, at'the carrierfrequencypr centre-frequency. For a constant i alternating current through the choke-coil -56. 1 the locus" of the voltages across coils 5'8 and 6.3

isgiven respectively by-' 'the circles-1V and n, as the frequency is deviatedjaboveand below the centre-frequency; For'asmallpercentage change 5 or the frequency from" centre-frequency, the

sharply resonant, circuit 59"60"w ill have the voltageacrossycoil 63 de-phas edby45'degrees; the

voltage across coil 63 is then represented by the currents flowing to the junction bf are unbal- For a frequency deviation on the opposite side of resonance or centre-frequency, this condition is reversed. There is thus produced across cathvariation 29.5'-per'cen't; when: is /2, variation is 19.0 per cent.,;--when-Na'is iin variation is- 7.6

.per cent; when N .is 2, variation is; 2.2 per; cent;

andwhen' N'is 3; the variation is=1-.0wper'cent. When the ratio -N. whichisernplo'yed, is sufficiently largerthe totaliunidirectional currentcan thus be made independent Iofflthe operating; freduency-deviationsupon the frequency-demodulatonto the extent-required; .W'henfrequencyvector fi'3e, and likewise the voltage across coil 58 is. then represented by the vector =58e. The resultant voltages across the diodes are now'rep- 3 resented by voltage vectors Brand E2, and 1 the j anced, there being greater unidirectional current 1 5 through resistance 21' than throughresistanceZB.

odes i k-26 a unidirectional voltagefwhich varies in dire ct .proportion to frequency deviations from centre-frequency over a useful range, the volt- .age. being zero at centre-frequency, and of opposite polarities for frequency deviations above and below centre-frequency.

quency deviations employedfor frequency-modula'tion. An important feature of this invention .is the means for obtainin independence of this total. unidirectional current from frequencydeviations,. over the entire useful range of such deviations in a frequency-demodulator, and for obtaining a. unidirectional controlvoltage there- .from, directly proportional to the amplitudes of the" alternating current of the input circuit.

Thus, it will be seen from Fig. Fl, that while the resultant-voltage across cathodes l4-28 due to frequency deviations is determined by the arithmetic. difference of voltages E1 and E2, for

any particular frequency deviation, that is, by the difference in unidirectional currents through the equal resistances. 21, 28,, the sum of these unidirectional currents is determined by the sum of the voltages. E1 and E2. ,The variationof, this sum with frequency-deviation, can be illustrated from Fig. 4; in reference to a voltage ratio employed; thatis,v the ratio-of alternating voltage across choke-coil 56 toalternating voltage across the resonant circuit 5960,. at the centre-fre 60 quency. Thusthe vectonA is a. measure of the half-voltage across resonant. circuit 59B0 when the junction 69 is at half-voltage across coils63 and B4; in series. Comparison can then be readily deviations are less than those su-fficient. to dephase the". resonant 5 circuit voltages by -A15 'degrees' theflpeicentage-variations "will be somewhatlessg butcsincetlie frequency deviations must" be a substantialpercentage of; this magnitude' to' yield adequate eonversionrvoltages between cathodes l 4' and 26.1 it apparent that the 7 ratio- N 1 .0}: defines a minimum f'ratio; below which the total uriidirectional .current wouldmbe unsatisfactory for control I purposesto compensate variations" of input alternating current: am plitude, or the'eifect of suchyariatio'ns upon the frequency demodulator, because" the unidirectio'n'al current cannot then-"be". solely dependent upon the amplitudes of the: input alternating In this i'rivention', the resistanceil'! is connected inseries"with the totali'unidirectional current fromthe diodes; to produce arunidirectional control-voltage, directly proportional to the-amplitudes of the impressed 'alternating currents and independent of frequency deviations 'over'th'e entire range of such deviations employed upon the frequency-demodulator. This unidirectional voltage is irnpre's'sed'upon the'control-grid' l I of tube 1', and "produces" a compensating voltage for the effect "'of amplitude-modulation upon the frequency-demo'dulator this compensating voltage is developed across the terminals; 34 of the sec ondary i'of' transformer 39"'5; is designated herein 'a', and discussed-in further detail hereafter. Referring toFig. 1, Whenthe switcheblades. 54 andSSarewsetto contact Fz'and- Er respectively, the'alterna'tin voltage across-resistances land 2 "in series set: up by the-diiferentialvoltage across ethercathodes-i4+2li is-impressed uponthecontrol-grids-AB I and. 5 I of the push-pull amplifier. The: cathodes '46, 41,; are at a; positive potential with reference totheground-tfi; and the controlgrids=-48 and 5-] thus have a constant negative bias-voltage introducedby way of circuit for adiustmentiof. point with reference to the ma n. conductance. characteristic of the tube 4 tand .42.. .T1lbes=4.l and .42, .arepreferably selected ,to h'a've astraight-line, relationships-between the. mutual conductanceand the negative .bias

-volitage, .over asubstantial operatine range, tha is. with mutual conductance demade between the voltage'sum E1'+E2 for resonant frequency and-the, voltage sum E1+E 2 for frequencies. sufficiently deviatedfrom resonance to de-phase by 45 degrees the voltages across coils ;5B and 6.3; also as influenced by the ratio of volt- V 7 age 56e to; voltage Az-l-B, that is the ratio of voltage across choke-coil 56 to voltage across resonantcircuit. 59- 6]! at centreefreq iency, Designating this latter ratio by N, then the percentage variation of E1+Ez from E1 '+E2" as related to N, is approximately as follows: When Ni; /3,

. creasing ,uniforinly with increase ofnegative biasvoltage; .such. characteristics are present in numerous conventional pentodes'. If the mutual conductance is designated by In I917 the'pentodes 4 II and 42, thenithe rate of 'change of 'mutual con- .duciance g'm with neglative bias-voltage is can stant inthe operating range employed, and higher derivatives of mutual conductance gn'i'with negative" biasfvoltage are ""zero, over the operating r e- H From a quantitativefconsideration of the voltage difference e. g. from Fig. 4, as'r'elated to the frequencyfdeviation frorn'centre freguency, .designatedby to the amplitude of the alternating current through 'the"choke-coi1" [56 connected tothe diode-connected resonantcircuit,

designated by Is,.and tocompositecircuit constants of the demodulator networkgdesignated by K, the conversion-voltage. across cathodes;

version-voltage across the resistances l and 2 inseries is thus substantially identical with the voltage across diode-cathodes [45-26. i It willbe evident that the alternatingcornponent of voltage across diode-cathodes |l26-'can be alternatively applied to the terminals; F2-'3-\-F1 of the push-pull amplifier by transformer coupling, the

primary being coupledwith the diode-cathodes" l4 -2 B,"and theflsecondary having terminals connected respectively with terminals F1 and F2, and" a centre-tap connected to terminal 3, in a manner well understood. 1

The total alternating voltage "across resistances I and 2, is thus ErQ-half of this' 'voltage being across resistance l, and half across'resistance 2 these'voltages being inthe samedirection, so that when this voltage produces a'positive polarity on" one control-grid, it produces "a negative-polarity on the other control-grid, and vice versa, The

compensating age e introduced at the ter:

minals 3 4, is common-to both control-grids 48 andil, and produces" the same polarity on both control-grids. With reference 1 to the resultant voltage applied tofeach of the control-grids 48 and. 5l, it is evident thatthe suIn ,of -voltage e; and voltage Er/ 2 is applied to on'f offthese vgrids andthe voltage difference e g-5:7 2 is applied to the other grid. The" resultant voltage"developed across the terminal s-t of the resistance 22 -24 inseries, is thus determinedfby the difierence oi the anode currents through resi'stancesfzz' and 24, and the magnitude of theequal resistances 22 or 24; Condensers 2i and 20'have negligible'impedance for the low (ii-demodulated frequencies, in relation to resistances IT or 18, and the output voltage of the amplifier atjtermin'als I6 l9 is substantially the same-as the voltage across s-' t, the continuous component" of current" being' ab sent from resistances I'Ijand I 8. "Inaccordan'ce with the well known relation in such a balanced r'nodulator, thevariationclirrent through one of theanodes is" l a1(ea+E1/2) +a2(ea+E//2)v and throughthe other anodef a1(ea- E;/2) +(12(6aEj/2) I 'The difi'erence' of these'anode variation currents is:

wherein the coeflicients a1 and (12 are respectively proportional to the mutualconductance gm, tube and circuit constants, and to the rate of change of gm with negative bias-voltage, tube .andjcircuit constants. 1

It will be noted that the resultant output-voltage across terminals Iii-l9 differs from the output-voltage. E1 across :terminals F11 and F2, by-the cross-modulation factor determinedby thecompensating voltage ea, as indicated in-Equation' 2. Asindicated by, Equation 1, v the output-voltage of the frequency-demodulator atterminaIs -F1 and F2 .dependsuuponrboth the, amplitude of alternating'input'icurrentsv Ia, and upon the frequency deviation A3. The resultant ,output-volt age across terminals l5-I 9, in=v iew oi Equations 1 and 2 is therefore;- i v i -KAf-l tl l-kealf' inwhich is a constantfwihichatalge 's into count constant K, and also constant a1 and the resistance 0 122 andconstaht 1c 'takes into ac-' count the ratio (ii/a1; Thus' jfor' proper compensation of'th'e effectoffchang'ing amplitudes of alternating ,input current le," it is evident that whenfIa 'is equal tothe norm selected, designated by-I O, the compensating voltage ea must'be z'ero';'also when Ia 'incfreaSGS above'the norm ,ee must be negative, and when 11a decreases below thenorm' ea must be positivegand furthe'r,that the magnitude'of changes of et, in

relation to I jmusit be such asfto maintain the product I(1 +ke u) constant, and equal. to the selectednormIee. The provision of such a vcomp en'sating' voltage is an importantfeature of this invention. The nature of the required'variatioh oi the compene' sating voltage is" indicated" by the necessary ec' ualityz' u I Ia(1+ka lac I or e (I /I rl) (3) The cathode of compensator tube 1 ,ean be set at a voltage which is positive with reference to the ground [5, 'by' rneans'ofthe adjustable contactor 14 upon the resistance 13. The "circuit: from cathode {to control-grid] l", is" 9 6j'1 4 -f|3- |sj 31 3 s 'j| it will be noted that the total? unidirectional current of "both "diodes flows throughresistance 3], in the direction f rom con-i and thatfthe control-grid is placed atfa 'iurther negative voltage by the'flow or total unldirectional current throug'h' resistance 31; n 'in n 'eon-f crease of negative bias-voltage uponf control-grid H, and; likew se a decrease ,of' current L below current norm'jlag produces a 'proportionatefde crease"'of negative bias-voltage; b'el'ovvfthat" of the norm. Corresponding current" variations are set up in'fthe 'anode circ'uit '9]35a .'35}%93 '2:- f|4-- :96,-* 'Ifhe variable resistance '35 "to regulate the voltage impressed upon" the of the transformer 35-5; the resistance,- 35 can be; made fsiiiiall in relation to the lanode -cathode resistance of ,thelpentode 1,] and 'in.,relation to the reactance or thejprirnary a'geveatne ra ge of frequencies for which compnsation pf tuna tions 9 alie ea ii u entemplitudell si ed. and th m e ln -vol se 81 0f t e se n ary 5,;rnadevto vary in responsetojcurren't: Ia,

that is with the i totall-iunldirectional current through resistancetfl'lr Referenceto the necessary equality whichmust be maintained; Equal;

Mei

tion indicates the nature oi mu-tua l-conduct' characteristics required "to" accomplish "this; equality, for purposes of comeensst mmss dis-- losed. ReferringtQ'lE 3, {911319 is indicated a graph of mutual-conductance of compensator tube .Iflfin 'relationto'" the origin yo the magnitu'desjqf mutual-conductance e -sung indicated along the axis 'oftbrdinatesfthe abscissas indicating'the magnitudes of negative bias-vo'ltageE 'fupo'n the control-grid il produced by "the unidirectionaltotal diode currentflowing through resistance 31, and with reference to the origin the j origin "CV, the ratio of any ffnegative "bias voltage Eg to'anynor'm of negativebias-voltage E o, .is the same as the ratio of theurespective" alternating currents, i. 'e.fIa /Iu= E /E and as previously noted, 'the compensating v voltage .ea' is directly proportional to'fthe variation of current through anode 91" Thus'the equality'indicatd by Equation 3 yields the tube characteristic re-' quired, in the form:

1 (E o/E12 in which i is .the magnitude ofthe variation-current through' the anode 9, and 'jlc" takes int'o account the constant 7:1, as wiellas the'constant proportionality of-es with the anode variation-cur re'zitith'e variation-current beingthe difference between 'any'cu'rrent'to' the anode 9 "and the current ito' the anodeiswh n the negative-bias voltagainst the norm Ego. The mutual-'conductance' characteristic best suitable for such con tria follows "from the "deflnition'of this quail-L t'ity, which 'is'w'eu. known, and'indicates that mutual conductance Gm should vary as the in; verse square otfllthe negativelbias voltage upon control-grid I'l,'6r approximately in'fthiSQmari-j ner. Such a ,characte fisticiis obtainable in numenus-conventiona pentddes, referred to 'as super-control" tubes, or variable-mu tubes, and gifd bias in addition to thatafiordedfby resistance 31 for control purposes, is providedltobetter attain"these}ob'jects. f

It"vvillbeunderstood that the quantitative relations'bointed out herein, are intended to aid in tlie'better underst nding of the principles. of

this invention/and arenot to be understood as any'manner'off limitation upon thestructures dis:

I It should be noted that .Equation l is for con ventional typs of frequency-demodulatlng cir cuits, armrest the amplitude of the received al ternating currents is involved as] a. ffactor deter mining-thefrequency-demodulated voltage Er. .It willlthus be evident, for -eXa'mp'le'that the simple superposition of modifying' voltages upon Er through the use of a single thermionic tube in which both and the modifying voltages are applied to grid means ofthe tube, cannot vannul the undesired modifications bill: for any appreciablerange-ot amplitude'modulations, in the sense (factually compensating forsuch 'magni tudes oofrandom variations, by 1 means of such modifying voltagesvaryingin-a direct proportion to altemating current amplitudes; the receiving apparatus has no control whatsoever over -th'e extent of such-amplitude-modulation; Further, the mere superposition or modifications simplybecause they-are in adirect proportion to lampli-- tude'variationsvvlll equally well'result in overmodifications of thesame-orderiofmagnitude as the original undesired arTbitna'ry modifications, for

any appreciable range of. amplitude variations. The corrective processes calledior, require a multiply-ing factor; :processes ofdirect additionor deduction as above noted, lead to the undesired .re--

sults stated. I The present invention is not concerned with such arbitrary counter-modifications of vthe'lampii tudemodulation, which necessarily ovens-modify for a very substantial and useful, range of amplitude-modulation, over which the controlineans in each tube oi a two --tube {pushpull amplifier, when such voltages are modified in a mrest-proportional relationlto .amplitudevarialtions of thereceived alternating currents. With further reference to arbitrary counter modifica- ;tions, it (is-evident to anyoneskilledinthe thatssince undesired modifications :of Ex occur in relation-to altern ating, l current amplitudes as the cause, counter-modifications if employed at all, .must necessarily be in relation to these a1.-v

ternating -current amplitudes; likewise the super position of arbitrary magnitudes .of countermodific'ations throughgridcontrd meansof thermionicltubes, is well understood in art. It is also wellrknown by those skilled: in the art of physical applications, that.however intricate .a functional relation maybe, itcan be matched over a suiilciently'infinitesimalrangeJoya simple additive' of subtractive variation. f Howl'evenlthe range of; randomamplitude' variations and corresponding undesirable variations .omusnot under conn01; QTY-11.11118 receiving apparatus, being externally.

determined and consequently. such a hi ly restricted range as above-.notedlis of llittle practical value, espe'cially since a wi'de range of random undesired modifications can then produce -,ov.ermodifications of the voltage ,Er comparable with the undesired modifications existing. with no corrective apparatus. An imDQrtant ffeature of the present invention is the meansproviding acrossmodulat'ing factor which'fvaries inversely with the amplitudes of received'altemating currents, .mak ing possible the actual compensation of random amplitude variations over a'wide range, and the simultaneous utilization, of both amplitude-demodulation voltages and frequency-demodulation voltages.

Whenthe switch-1119416554, 55, are switched, to contacts or terminals A2" and A1 respectively, the compensatingvoltage ea is eliminated, the output-voltage across resistance 6 due to amplitude-modulation being applied across controls'rid fii and-cathode 41; Terminal A2" "is connected-by VariabIe connector 23, to the outputres'istance 18,101 phase-inversiom'to apply to controlg rid fl an equal-voltage of opposi'te phase tothat applied to control-grid 5|, by th'eresist ng -s, I.

It will benoted that when switcl i -blades-54 and 55ers connected respectively to tenminals Fz and F1; irequency-demodulation compensated for the effect thereon; ofamplitude-modulation; is available" -"at vthe itemnnals ill -l9, and Ithatxsimultaneously;amplitude mbdulatiomis available at terminals.-A1- -l15, andican I be :taken from these terminals .directly- :or. ibyzanother. amplifier;

2 882 O1 6 The above-disclosed demodulators thus provide simultaneous reception of both the a plitudemodulation and the frequency-modulation from an impressed carrier-voltage having both of these modulations; also these demodulators afl'ord a useful control of devices interconnected between the output-terminals for frequency-modulation and the output-terminals for amplitude-modulation, through the medium of phase-control, or amplitude-control, or both, with'reference to the low o modulation-frequency impressed upon the carrier, that is, upon both the amplitudes and upon the centre-frequency.- For example, if a low frequency in is impressed upon the carrier so as to modulate both the amplitudes of the carrier and the centre-frequency of the carrier, a selective control is provided by the above-disclosed demodulators and for devices interconnected between output-terminals, by control of the phaseshift of the modulating-voltage of frequency fr. upon the amplitude-modulator in relation to the modulating-voltage of frequency 1; upon the frequency-modulator. In reference to the T-connection formed by terminals I6-jl9,A1, and when the terminals F1 and F2 are respectively in contact with switch-blades 55 and' 54, it will be noted that when these modulating-voltages are in phase, and the demodulator output-voltages are adjusted to equality,'for example by taking the terminal A1 from an intermediate position upon resistance 6, or by taking the terminals l6, ill, from equal intermediate positions along resistances I1 and I8, respectively, in'a manner well understood, then zero voltage is established'jbetween a pair of interconnected terminals, such'as A1--l6, and double voltagle'is established between terminal A1|9. When the same modulatingvoltages are shifted to exact phase-opposition, then zero voltage isproduced across terminals A1-l9, and double voltage across terminals A1l 6. When the phase shift between the modulating-voltages is varied, and has values between the in-phase and phase-opposition positions, various degrees of output-voltage magnitudes across these output-voltage terminals are produced, and controlled by the controlled composition of the carrier-voltage, as to the phase-shift." There is thus provided, these additional means for the control of devices connected to the outputrterminals of the demodulators, and for the reception of signal-intelligence. As a furtherexample, it will be noted that if the output-voltageirom terminals A1l5, or from another amplifier connected to this vo1tage,fsuc h as is illustrated for the con nections A1, A1, is proportioned to 8616 percent.

of the output-voltage across IE-19, and the phase-shift is 90 degrees, then in addition to the production of' device-operating voltages across terminals l6-l 9, and I5A1 or the amplifier output-equivalent of terminals'l 5'-A1', thereis produced a three-phase system with reference to the terminals 16, I9, and A1. As is well known, such a three-phase system can be readily em ployed for the production of a rotating electric or magnetic field for controlpurppsesgit being evident that the direction of rotation is controllable by the particular 90 degree phase displacement chosen, and the speed of rotation by thefrequency fL which is employed, v I

In the Figures '1 and 2, condenser 40 has a negligible impedance at the low or demodulated frequencies, in relation to resistance 6; also condensers 29, 30, and 38, are intended to by-pass the high or carrier-frequencies around the respec tive resistances across which theyare shunted,

and thus have-negligible impedance atthesefrequencies in relation to the respective'resistances which are shunted; Condenser I3, Figz' 2', negligible impedance at the ourier-frequencies;- in relation tothe impedance of the choke coil'ibd, sogthat substantially the full input alternating is impressed voltage across terminals 10w85, across choke-coil 56a.

Having thus described several illustrative em'- bodi'ments of my invention, it will be evidentthat changes can be made in the form and arrange ment'of parts without departing fromthe spirit of my invention, as set forthin the appended claims, and I do not therefore limit' the scopeflof the invention to such particular embodimentsfor; by the terms of' the appended otherwise than claims.

What is claimed is:

1. A demodulator providing separate frequency-demodulation output-voltages and ain-- plitudedemodulation output-voltages, simultaneously, from an impressed high-frequency carriervoltage, said demodulator having a thermionic double-diode, a, resonant circuit, connections between said double-diode and resonant circuit in cluding a first resistance connected between the cathodes of said double-diode, a connection from each anode of said double-diode to said resonant ance to said resonant circuit; a ground connection at said half-resistance point, and thermionic tube compensating means having a control-grid to cathode control-circuit including connections between said second resistance and ground connection, compensating the effect of amplitude variation upon said frequency-demodulation "output-voltages. 2. 'The combination with a demodulator for demodulating constant-amplitude frequency-modulatedalternating currents, of a compensator device, said demodulator having a double-diode, a first resistance connected between the cathodes of said double-diode, a resonant circuit having an inductance coil, a conductive connection between the anodes of said double-diode including inductance of said coil, and a choke-coil connected from the half-resistance point upon said first resist ance to the half-inductance point upon saidineluded inductance between said anodes, said compensator device having a second resistance connected in series with said choke-coil, between said half resistance point and said choke-coil, a thermionic compensator tube having a cathode, anode, and control-grid, a ground connection at said half-resistance point, connections between the compensator tube control-grid and compen sator tube cathode including said ground connection and a part of said second resistance, and a a first resistance connected between the'cathodes of said double-diode, a resonant circuit having an inductance coil, a conductive connection between the anodes of said double-diode including inductance of said coil, and a choke=coil con nectedin series witha second resistance; due ter tionupon said Lpfu minalof said choke-coil being connected tq; the hal-fqnductance .point'; upon said included induct;

ance hetween said anodes, and one fte'rminal'i of aid. ascend, tan e ei mnnecied he aliresista ce point upon said first resistane and tg the ground, a th'emnionic compensator tube havinga cathode, anode, and .c ontrol -gizid, connections between the compensator tube'oontrolegifid and cathode including said groundfcon nection and a part of said second resistance, and a balanced modulator including aj'thermionic push-pull amplifier having Iafirst and second control-grid an. impedance connected between said first vand 1 second control-grids coupled "with" saidlfirstresistancel andcoupling means coupling the. alt-impedance 'poin't, upon said impedance with the compensator tube anode-cathode.

vice haying a resonant .c'ircui't, diode rectifying me nstoaauc ave y connected to said "resonant circuit; a twof'tub'e push-pull thermionic amplifier havinghn outpu'tfdmpedance-for rrequemy-de modulated voltages, an input-impedance con; nected; withrgrid means in each of saiditubes, and a first'cb lnling means. couplings-aid input-impedance with said diode means, impressingalter natingjvolt igs of opposite polarity'upon saidg id meanjsgan outwit-impedance for amplitudefdmodulated voltages; a second coupling means coupling fsaidj output-impedance for amplitudedemofiulatedylta'geswith said diode'j'mea'ns, a.

thi'i'd'coupl'in'g flmeahs including a grid-controlled thermionic lconductance coupling the half-impedancefpoin't upon said rinput-nnpedanceiwith said res'sing'coinpensajting voltages v a t I ponj said gri means ina rec'ip rocalsrelationship. to the magnitudes of said am plitudeemodula'ted Voltafies; and a" common connectionlbetwee'n'said output-'impedances, whereby said output-voltages are'serially combined;

"G QIn Ea' IfIeQuen 'y demOduIatOr; a ,doublQ-diod, a. first resistance .connectedfbetween the. cathodes of saidld oubl e -diode, .a resonantcirouit having an r inductance" coil, Iaconductiue connection between theanodes of said double-diode eluding inducte ancelof vsaidj'vcoil,,a,connection irom the half inductance point uponsaid included inductance hetweensaid anodes to the half-resistance point uppni said first resistance including smoke-[c011 and .a's'econd resistance in series, .said second "resis'tance having one-terminal. connectedlto said half-resistance point, counlingmeans coupling said resonantfcir'cuit and said choke-"coil witha source of 'inodulatedl alternating voltages, and a groun jcon'n'e'ct ion' at said ha jnresisiance QpIoint upo n'sai'd'first resistance. I f

' Incon hination with the structure or claim '6 the mi nio; 'icontroL-mean's, ncluding 'afjca'thQde and a lcontrol-grid' and, circuit man anese-J hai e is alih a dfifee rii i r dile?- mionic control-mean including said ground con- Iitidnand.saidsecon'd resistance.

BlIjfIri ico'in'bin ationwith the structure of claim 6,

a balanced-madman including a f resistance- 1m 'inan'fs c u ling "said second resistance"with' means c upli g saji'dgnrstanasecondeontro anns wit'h'jsaid ,.fi r's t"1' esis'tance, "and a third coupling means coupling a'tljird resistance with said groun connection and with said second resistance, whereby frequency-demodulation is provided across the output circuit of said push-pull amplh fien' ndependent oi amplitude-modulation oi said modulated iij/o1tages,.andwhereby demodulation of said amnlitude-modula'tion is simultaneously provided across said third resistance, independents: the d tae I flO. A demodulator f'de'vic'e', said, device having, a double-diode, a first resistance connected between the cathodes of said double-diode, a'resonantcirf cuit'havingjan mductancejcoil, a conductivecon nect 'ion between'fthe'anodesof said double-diode including finductance offsaid coil, a connection from th'e,halflinductance point .uponjsaid ineluded [inductancefhetwe'en' said 'anodesft'o' the hair-resistance point upon said firs'tresistance, including a choke-coil and .a vsecond resistance in series; said second resistance having one termi 'ialfldonnected to's'aid half-resistance point, a grjou 1 Y connection "at said halferesi'stance point upon said firstres'istance; coupling means counling said resonant circuit and. saidchoke'- coi1 with a source' ofmo dula'tedalternating voltagesjand 'a balanced-modulator including "a, resistance-'coupied push-pull amplifier 'havinala first fco'ntrolgrid and a second control-grid, a second coupling means coupling said second resistance with said first and second control-grids,"and a thir d' coupling means coupling 'saidiflrstand second control-gridswith sa'idfirstresistance. I I '11."In combination withlth'e structure of claim 10,1a fourth coupling means, coupling athird resistancew'ith said jgroun'd. connection and with Said second resistance, and selector'means'set v y connecting said'fi'rst'and second control;

' grids'respectirely with said third resistance; and

with a phase-inverting connection upona'n out put resis'tan'ce ofjsa'id amplifier.

' a frequency{de nOdLIJ'atQr for demodulat ing' jfrequency niodula'ted alternating currents, thede vlce for stabilizing the factor of alternating current magnitudes in the conversion ratio of frequency-deviation to outputvoltage, independent" of "random ,va'riationsof amplitudes o'ffs'aid alternating currents, said demodulator having a resonant circuit, and diodevoltage", rectifying means including a resistance coupled with' said resonant circuit, providing unidirectional voltages uponesaid resistance in adirectproportion to amplit'udes or said alternatin currents and'substantiaily independent of frequency-modulation; said d 'vic including a two-tube ip'us'h pull amplifier, each tube .nav'ing'a cathode, anode, andgrid control means for controlling cathode-anode-currents "binatifon With the structure-of claim 6,

frequency-modulation of" said modulated therein, said amplifier having an output-impedance for frequency-demodulated Voltages coupled with said anodes, an input-impedance connected between said grid control means coupled with said diode means, and a thermionic compensator tube with anode, cathode and a grid, having a first coupling means coupling said compensator tube grid and compensator tube cathode with said resistance, and a connection between said inputimpedance and said cathodes of said amplifier, including a second coupling means coupling said grid control means with said compensator tube cathode-anode, impressing voltages of like polarity upon said grid control means and providing a'compensating cross-modulating factor Varying in inverse relation to the amplitudes of said alternating currents;

13. The combination with a frequency-demodulator for demodulating frequency-modulated carrier currents, said demodulator having the capability of providing output-voltages directly proportional to frequency deviations from a centre frequency, and directly proportional to the amplitudes of said carrier currents, of a compensator device for compensating the undesired efiectsof amplitude variations of said carrier currents upon said output-voltage, said device having a two-tube thermionic push-pull amplifier for amplifying said output-voltages, and compensator means for controlling a cross-modulating factor of said pushpull amplifier, including a thermionic compensator tube with cathode, grid, and anode, a first coupling means coupling said compensator tube grid-cathode with said demodulator, and a second coupling means coupling said compensator tube cathode-anode with grid controlling means in each of said tubes of said p -pull amp fier- 14. A frequency-demodulating device for demodulating frequency-modulated carrier currents, said device having a resonant circuit, diode 15. In a frequency-modulation receiver having a frequency-demodulator with a resonant circuit and diode voltage rectifying means including resistance connected with said resonant circuit, and a two-tube thermionic push pull amplifier for providing a substantially constant. voltage amplification-ratio, each tube having a cathode, anode, and. grid control means for controlling thermionic currents therein, said amplifier having an inputimpedance connected between said grid control means, an output-impedance connected between said anodes, and a'coupling means coupling said input-impedance with said rectifying means, a compensator device for compensating undesirable modifications of said frequency-demodulated voltages caused by amplitude variations in th impressed carrier wave, said compensator device including a thermionic compensator tube with compensator tube cathode, compensator tube anode, and compensator tube grid, a circuit connection betweensaid compensator tube grid and compensator tube cathode'including a coupling with said resistance, and a common circuit connection between said grid control means and said cathodes of said two amplifier tubes, including a couplin with said compensator tube anode and compensator tube cathode.

16. A frequency-demodulator having a resonant circuit, and a two-tube thermionic push-pull amplifier for providing a substantiall constant v01t age-amplification ratio, each tube having a cathode, an anode, and grid control means for con-,

. said resonant circuit and impressing voltages of voltage rectifying means including a resistance coupled with said resonant circuit, and a twotube push-pull thermionic amplifier, each tube having a cathode, anode, and grid control means for controlling the cathode-anode thermionic currents therein, said amplifier having an output impedance for frequency-demodulated voltages connected between said anodes, an input-impedance connected between said grid control means and coupled with said diode means, a thermionic compensator tube with anode, cathode and grid, a second coupling means coupling said compensator tube grid and compensator tube cathode with said resistance, and a common connection between said grid'control means and said cathodes of each of said two amplifier tubes including a third coupling means, coupling said grid control means with said compensator tube cathode-anode.

opposite polarity upon said grid control means, and a compensator means for impressing comev pensating voltages of like polarity upon said grid control means and regulating the magnitudes of said compensating voltages, including a thermionic compensator tube with compensator tube cathode, compensator tube anode, and compensator tube grid control means, a common circuit between said amplifier tube grid control means and said amplifier tube cathodes including a coupling with said compensator tube cathode and compensator tube anode, and means for impress ing voltages upon said compensator tube grid control means including a connection between compensator tube grid control means and said resistance.

EDWARD H. LANGE. 

